1. Field of the Invention
This invention pertains to a screw driver exclusively applied to drive a specialized screw having compound drive recesses as shown in FIG. 20 of U.S. patent application Ser. No. 09/193,489, whereby a screw can be driven by various conventional drivers, i.e. slot, Phillips (cross), square-cross, square, and double-cross recesses, particularly to the screw driver with a drive bit provided with four fan-shaped blades symmetrically disposed with respect to each other, between every two neighboring fan-shaped blades and has an elongated rectangular hollow indented space, a square blunt bottom protrusion formed and situated at the center section of the free end of the drive bitjoining the four fan-shaped blades and the four hollow indented spaces.
2. Description of the Related Art
Conventional fastener drive recess types include slot recess (A) shown in FIG. 1, cross recess (B) shown in FIG. 2, cross-square combination recess (C) shown in FIG. 3, square recess (D) shown in FIG. 4, and double-cross recess (as available under the trademark POZI) (E) shown in FIG. 5. Drive recess structures of aforementioned types generally cannot stand comparatively large torque due to their structural weaknesses and uneven pressure distribution. Typically, if a screw resists turning, then upon applying an increased torque to the driver bit, it slips upwardly out of the drive recess. The more frequent this slippage occurs, worse damage is imposed on both the drive recess and the drive bit. Loose engagement (the drive bit wobbles excessively within the drive recess) is one of the major aspects to consider when both the drive recess and the drive bit fail to withstand a large driving torque.
Five kinds of screw driver bits are shown respectively in FIGS. 6, 8, 10, 12 and 14. In FIG. 6 a slot drive bit is designed to drive recess A (as shown in FIG. 1), and FIG. 7 shows how the slot drive bit is engaged in the compound drive recess. On the sample principle, in FIG. 8 a Phillips drive bit which corresponds to recess B (FIG. 2) engages within the compound drive recess as shown in FIG. 9. In FIG. 10 a cross-square drive bit which corresponds to recess C (FIG. 3) engages within the compound drive recess as shown in FIG. 11. In FIG. 12 a square drive bit which corresponds to recess D (FIG. 4) engages within the compound drive recess as shown in FIG. 13. In FIG. 14 a double-cross recess (as available under the trademark POZI) drive bit which corresponds to recess E (FIG. 5) engages within the compound drive recess as shown in FIG. 15.
Although the screw driver bits, as shown in FIGS. 6-13, are capable of driving the aforementioned compound drive recess, they all fall short of being adequate to either maintain a best possible retention between the drive bit and the drive recess, or supply the maximum driving torque to the aforementioned compound drive recess. Hence, a specific screw drive bit with four fan-shaped blades for the aforementioned compound drive recess is devised to achieve their maximum performance.
Next, the screw drivers disclosed in U.S. Pat. Nos. 2,764,197 and 3,913,647 are specifically designed to fit in recess B (as shown in FIG. 2 of U.S. Pat. No. 2,764,197) but not for the aforementioned compound drive recess; therefore, the maximum driving torques the above two screw drivers can supply will be less than this present invention when they are applied with the aforementioned compound drive recess.